NOTES: EPA no longer estimates
lead emissions. Modal shares in 2000 were: highway gasoline vehicles, 4.1%;
aircraft, 95.9%. Other includes recreational boats,
airport service vehicles, and road maintenance equipment. To make it easier to compare data of differing magnitudes over
time, the Bureau of Transportation Statistics divided the data for all years in
each category (see table 15-1b in appendix B) by the initial year value and
multiplied the result by 100.

SOURCE: U.S.
Environmental Protection Agency, Office of Air Quality Planning and Standards, National Emissions Inventory, 2002 Air Pollutant Emission Trends, available at http://www.epa.gov/ttn/, as of March 2005.